Field of the Invention and Description of the Prior Art
The present invention relates to the area of procelain frits, their preparation and use, and especially to dental procelain frits. Dental porcelain frits are useful as the raw material for preparation of procelain denture teeth, reconstructive jacket-crowns and inlays, and restorative constructions over metal or preformed alumina substrates.
Despite the considerable efforts of dental manufacturers, current commercial dental procelains are characterized by high fabrication costs, significant fabrication difficulties and severe performance problems. The fact that the highest paid worker in a dental laboratory is usually the ceramist suggests the difficulty of dental porcelain fabrication. The properties of dental porcelain which result in these fabrication difficulties include (1) firing shrinkage, (2) flow deformation or slumping, (3) stress buildup in the porcelain body (usually as a result of mechanical incompatibilities originating at the porcelain-to-metal interface), (4) flaws and incipient cracks commonly generated within the porcelain by grinding, and (5) warp.
Firing shrinkage is the single greatest factor complicating the processing of restorative dental procelains and is a factor in the failure of porcelain-to-metal bonds. The firing shrinkage of various porcelains under differing conditions ranges from as low as 27 to as high as more than 40 volume percent. Repeated rapid firing and cooling to add to the porcelain and thus compensate for this shrinkage subject the dental construction to repeated thermal shock which results in uneven stress buildup in the porcelain and contributes to stress accumulation in the total metal-ceramic unit.
Flow deformation, the second important problem in porcelain fabrication, is apparent in the edge rounding, central massing and nonuniform shrinkage of the porcelain. These characteristics make shoulder construction and marginal adaptation difficulat and contribute to the difficulty of porcelain inlay and jacket-crown fabrication. Ordinally, the central massing problem can only be reduced by scoring the construction. To compensate for flow deformation effects, it has been common to overbuild, especially shoulders, which in turn requires grinding back and re-firing.
Flow deformation has been minimized to some extent in a few commercial body or dentin porcelains, but apparently not effectively with enamel and incisal porcelains. One of the most commercially successful porcelains for metal-ceramic constructions (Ceramco, Johnson & Johnson) exhibits severe edge rounding caused by flow deformation. This rounding may be alleviated by proper application of a liquid additive consisting of a water solution of silica sol and small amounts of fluxing agents. See U.S. Pat. No. 3,973,970. The additive there described constitutes a minor proporation of what is other wise a traditionally prepared porcelain. However, it would be preferable to have form retention during firing inherent in the raw porcelain powder.
Stress buildup in the porcelain may be caused by a number of factors and may result in weakening or even failure of the porcelain-to-metal bond. Excessive base-metal oxidation can cause formation of thick surface layers of refractory oxides of chromium, aluminum and manganese causing failure in adherence of porcelain to metal. The firing of porcelain on metal at temperatures at which metal flow deformation occurs can result in a casting of poor fit. Ultimately, most stresses at the porcelain-to-metal interface appear attributable to the differential thermal contraction which occurs between porcelain and metal on cooling after firing. The stress effects induced in the metal-ceramic by variations in thermal history and dimensional compensations make it imperative to simplify and abbreviate porcelain fabrication operations, particularly the number of fires. This can only be accomplished if the problems which dictate an increase in the number of fires, i.e., firing shrinkage, flow deformation, and, to lesser extent, warp, are in turn minimized.
The overbuilding of commercial porcelain to compensate for flow deformation also generates the need for machining the fired porcelain. Grinding of dental porcelain introduces flaws which may or may not be satisfactorily healed by glazing and refiring. Susceptibility to failure introduced by cracks, flaws and brittleness is a serious problem with aesthetic dental porcelains. Even with careful fabrication, the tensile strength of ground, unstressed translucent porcelain is not particularly high for a ceramic material, even though it exceeds the strength of tooth dentin.
Current commercial dental porcelain frits are prepared from cyrstalline oxides, e.g., feldspar, quartz and boric oxide, by a process that requires complex mineral beneficiation, milling and blending procedures followed by high temperature fusions and calcinations involving remilling and reheating. The fusion step is followed by the supercooling of silicate liquids. As the liquid supercools, the composition passes through immiscibility domes where the viscosities are sufficiently low to permit phase separation. The resulting commercial dental porcelain frits are not compositionally homogeneous on a microscale. They contain quartz, relict (or partially digested) feldspar and other phases in various degrees of digestion and formation. It is believed that uniform fluxing of all porcelain size fractions aids in form retention. However, such iniform fluxing is impossible when the fluxing constituents are not homogeneously distributed, as is the case with commercial dental porcelain frits.
Furthermore, when porcelains prepared by traditional high temperature fusions are over-fired, they "age" and become too translucent, develop too high a gloss, may discolor, exhibit excessive flow deformation, and, if severely overfired, become too brittle and weak and dull in appearance.
Attempts have been made to remedy some of the problems encountered wth existing dental porcelains. Aluminous porcelains have been prepared and are said to improve rupture and compressive strength and to inhibit crack propagation. See McClain & Hughes, "The Reinforcement of Dental Porcelain With Ceramic Oxides," 116 Brit. Dent. J. 251-64 (1965). However, due to excess opacity, these materials are suitable only for the construction of cores of crown and inlay porcelain and are not useful as body or dentin porcelain in metal-ceramic restorations.
To strengthen porcelains, the introduction of compression into the ceramic via ion exchange with molten alkali nitrate has been suggested, but appears too complicated for the extreme simplicity demanded in the dental laboratory. See Southan, "Strengthening Modern Dental Porcelain by Ion Exchange," 15 Aust. Dent. J. 507-510 (1970). Also, sintered translucent aluminas are extremely strong with tensile strength easily in excess of 760 kg.cm.sup.2. See Linch, Ruderer & Duckworth, "Engineering Properties of Selected Ceramic Materials," American Ceramic Society 5, 4, 1-9 and 5, 4, 1-17 (1966). However, the prolonged firing times and high processing temperature required in the preparation of sintered translucent aluminas preclude their use except as preforms. See Ishitobi, Shimada & Korzumi, "Fabrication of Translucent Al.sub.2 O.sub.3 by High Pressure Sintering,"56 Cer. Soc. Bull. 10-16 (1977).
Glass-ceramics have been experimentally developed for dental restorations, and the strengths are very much greater than those of ordinary porcelains. See MacCulloch, "Advances in Dental Ceramics," 124 Brit. Dent. J. 361-65 (1968). However, the procedure involves casting molten glass into molds and heat-treating to generate the glass-ceramic. From a process point of view, this technique with its complicated "pyroceraming" to generate color shade appears too difficult for restorative use.
The instant invention makes available a new type of dental porcelain frit and dental porcelain prepared from that frit which avoid many of the problems of prior art compositions prepared by traditional fusion techniques. The inventive porcelain frit is characterized in that it is prepared by a gel route. Thus, the final porcelain construction is derived from an amorphous precursor, rather than from crystalline oxides. Preparation of dental porcelain by a gel route is believed to be an entirely new field of endeavor. An area of research which is somewhat related, but clearly distinct, is the general investigation of gel route prepared glasses.
Investigation of compositions formed by acidification followed by dehydration of glasses with oxide components of Li.sub.2 O, Na.sub.2 O, K.sub.2 O, Rb.sub.2 O, MgO, CaO, SrO, BaO, PbO, Ga.sub.2 O.sub.3, Fe.sub.2 O.sub.3, La.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2, and ThO.sub.2 has shown that these components are more homogeneous than the best glasses obtained by the usual techniques of melting solid oxide constituents, and that fewer meltings are required to achieve homogeneity. See Roy, "Gel Route to Homogeneous Glass Preparations," 54 J. Amer. Cer. Soc. 639-40 (1971). See also Roy, "Rational Molecular Engineering of Ceramic Materials," 60 J. Amer. Cer. Soc. 350-63 (1977). It is known that glass preparation by the gel route is faster, less labor intensive, and requires less energy than is needed for preparing glass from traditional crystalline oxide materials and that fully dessicated gels from which all water and nitrogen oxides have been removed by heating can be melted and cooled to yeild clear glasses 100.degree. to 200.degree. C. below the temperatures required for standard batch materials. See McCarthy & Roy, "Gel Route to Homogeneous Glass Preparation: II. Gelling and Dessication," 54 J. Amer. Cer. Soc. 639-40 (1971). See also Kuczynski & D'Silva, "Formation of Glasses by Sintering," 54 J. Amer. Cer. Soc. 51 (1971).
Other workers have confirmed that glasses made from gels in the system SiO.sub.2 --Al.sub.2 O.sub.3 --La.sub.2 O.sub.3 --Zro.sub.2 are more homogeneous than those made from crystalline oxide mixtures. See Mukherjee, Zarzycki & Traverse, "A Comparative Study of `Gels` and Oxide Mixtures as Starting Materials for the Nucleation and Crystallization of Silicate Glasses," 11 J. Mat. Sci. 34-355 (1976). This reference is directed to the investigation of properties of gel-prepared glasses and glass-ceramics. The gels are fused by complete melting at very high temperature (e.g., 2,000.degree. C.) and are then permitted to cool. It has been shown that the effective poor size of alumina gels heat treated to 500.degree. C. and above increases as the surface area decreases. It appears that the sintering between primary particles which causes reduction in microporosity does not reduce greatly the total pore volume. See Yoldas, "Transparent Porous Alumina," Amer. Cer. Soc. Bull. 286-90 (1975). Additional work in the field of gel prepared glasses is exemplified by U.S. Pats. Nos. 3,929,439; 3,759,683; 3,782,982; German Pat. No. 2,128,845; German Pat. No. 2,128,980; and by Fripiat & Vytterhoeven, "Hydroxyl Content in Silica Gel `Arerosil`, 66 J. Phys. Chem. 800-05 (1962); Vytterhoeven, Hellinckx & Fripiat, "Le Frittage des Gels de Silica," Silicates Industriels 241-46 (1962); Yoldas, "Alumina Gels That Form Porous Transparent Al.sub.2 O.sub.3,"10 j. Mast. Sci. 1856;14 60 (1975); Luth & Ingamells, "Gel Prepartion of Materials for Hydrothermal Experimentation,"50 The Amer. Mineral, 255-58 (1965); McCarthy, Roy & McKay, "Preliminary Study of Low-Temperature `Glass` Fabrication," 54 J. Amer. Cer. Soc., 636-38 (1971). For advances in the area of gel-prepared microporous glassy fillers for dental resin composites, see Mabie & Menis, "Microporous Glassy Fillers for Dental Composites," 12 J. Biomed. Mats. Res. 435-72 (1978), now the subject of U.S. Pat. Nos. 4,217,264 and 4,306,913.